I know the mechanism by which shear viscosity produces shear stress in gases; i.e. exchange of molecules hence momentum between two layers of fluid because of random thermal motion. But how does this process generate entropy and dissipate kinetic energy of bulk motion into random thermal motion of molecules.
Well, if you want to consider a molecular level, you can no longer invoke the continuum model and the differential equations that describes the flow motion. And, hence, the same viscosity is not what we are used to consider, as well as the Newtonian model can not be introduced as it adopts the velocity gradient.
In the general framework we consider that the kinetic energy is dissipate because in the governing equation the dissipation function acts and looking at the internal energy equation we see that this term acts to convert the mechanical energy in internal energy by increasing temperature and heat. This process is not reversible and is associated to an increasing in the entropy
A physical system always attempts to distribute its internal energy equally between all available degrees of freedom. Since the flow motion is very coherent then its energy at the boundary will be transferred into less and less coherent forms of energy over larger and larger numbers of degrees of freedom until all that is left is totally 'random' motions perceived as heat.
Thank you Prof Denaro for your answer. I think i did not put my question clearly. So I want to ask it again. I underststand that we cannot use concept of continuum level directly at molecular level. But we can explain the cause of a physical phenomenon at microscopic level which is happening at macroscopic level. For example, we have microscopic reasoning and picture for the pressure, heat conductivity, sound wave, mass diffusion in gases. We also calculate pressure in molecular dynamics simulations and say that if system contains large enough molecules then this pressure will be correspond to macroscopic pressure. Now, viscosity represents diffusion of momentum in direction of normal to flow velocity. It is a physical phenomenon, so we can seek for its explanation at microscopic level and we have one i.e. exchange of molecules between adjacent layers of fluid. My question is how does that process increases entropy of the system and how it transfers kinetic energy of bulk motion into thermal energy of molecules. Thank you very much again for your answer.
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